Kinetics, Microstructures and Mechanism of Internal Oxidation - Its Effect and Prevention in High Temperature Alloy Oxidation

Abstract

Abstract A review and evaluation of the role of internal oxidation in the oxidation of alloys is presented and five alloy types represented by Ag-In, Cu-Be, Ni-Cr, Nb-Zr and Cu-Zn-Al are considered in detail. Internal oxidation is a diffusion-controlled process for which the kinetics may be predicted from solutions of the diffusion equation. Except for oxidations involving the formation of extremely stable solute oxide precipitates or oxidations at temperatures allowing only minimal solute mobility, the experimentally measured kinetics are in agreement with the calculated rates. The radius of a spherical internal oxide precipitate is expected to vary directly with the depth at which the precipitate is formed and inversely with the oxygen solubility at the external surface. Upon exceeding a critical solute content in a given binary alloy system, the occurrence of internal oxidation of the solute is replaced by the formation of an external scale of the solute metal oxide. A reduction in the oxygen content at the surface or the introduction of deformation into the surface of the alloy can reduce the solute content required for the transition from internal to external oxidation. In most cases, the prevention of internal oxidation by the formation of a compact surface layer of the solute oxide will result in a reduction of the oxidation rate of an alloy. When internal oxidation of a solute occurs in combination with external scale formation, the morphology of the precipitates in the microstructure of the oxidized alloy is determined both by the precipitation conditions and the mode of external scale formation. Internal oxide precipitates can affect both the mechanism and kinetics of external scale formation.